Towards A Flexible Mediation Framework for Dynamic Service Invocations Philipp Leitner, Florian Rosenberg, Anton Michlmayr, Schahram Dustdar Distributed Systems Group, Vienna University of Technology Argentinierstrasse 8/184-1 A-1040, Vienna, Austria
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Abstract One of the main benefits of service-based systems is the loose coupling of components, which allows for flexibility in the selection of internal and external business partners. However, currently this flexibility is severely limited by the fact that components have to provide not only the same functionality, but do so via virtually the same interface. Invocation-level mediation may be used to overcome this issue – using mediation interface differences can be resolved transparently at runtime. In this paper we present the general concepts of invocation-level mediation, and show how these ideas are integrated in our dynamic service invocation framework DAIOS. To demonstrate the flexibility of our mediation framework we have implemented two fundamentally different mediation strategies, one based on structural similarity and one based on semantically annotated WSDL (SAWSDL). We evaluate the runtime performance of our mediation strategies, compare them with unmediated invocations and relate the overhead introduced by invocation mediation to the flexibility gained by this approach.
1. Introduction Systems based on the Service-Oriented Architecture (SOA) [15] decouple clients from the service providers they are using by utilizing standardized protocols and languages (HTTP, SOAP [22], WSDL [23]) and a registry such as UDDI [20] as service broker. In theory, this loose coupling allows service clients to roam freely between internal and external business partners, and always select the partner that is most appropriate at any given time. However, in practice this flexibility in the selection of partners is currently severely limited by the problem that clients rely on specific service interfaces for their invocation – therefore, services need to adhere to identical WSDL contracts in order to actually be interchangeable at runtime. The assumption of interface compatibility is of course not realistic if services
are provided by different departments or companies. Currently, most work in the area focusses on providing an additional infrastructure to resolve these compatibility issues: ESBs [17] provide an additional bus that decouples clients and services, and integration adapters or mediators [1, 18] are used as intermediary to resolve the inherent problems of invocation heterogeneity. The approach that we present within this paper follows a different idea: we use a pure client-side approach to mediation, i.e., we enable the clients themselves to adapt their invocation to specific target services. Specific mediation behavior is introduced in the clients using mediation adapters, which can either be general (e.g., a SAWSDL-based [24] semantic mediator) or tailored towards specific domains or scenarios. This lightweight approach removes the need for an explicit mediation middleware, and resembles the traditional idea of SOA (where clients and services interact directly) more closely. The contributions of this paper are fourfold: firstly, we summarize the general concepts of dynamic invocation mediation; secondly, we present how the existing DAIOS Web service invocation framework has been extended to include a dynamic mediator interface, thirdly, we explain the implementation of a set of initial mediators that exemplify the capabilities of this interface, and finally we present the results of an initial evaluation of the DAIOS mediation interface. The rest of this paper is structured as follows: Section 2 clarifies the need for invocation mediation based on a illustrative example, Section 3 elaborates on some related work in the field, Section 4 explains the general concepts of dynamic invocation mediation, Section 5 details the DAIOS mediation interface and the mediators that we have implemented using this interface, and Section 6 shows the results of a preliminary evaluation of these adapters. Finally, Section 7 concludes the paper with some final remarks, and an outlook on future work.
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